Add unset_variable()

This should be useful for helping to control variable scope within
Meson. CMake has something similar for controlling scope.

data/syntax-highlighting/vim/syntax/meson.vim

@@ -3,7 +3,7 @@
 " License:	VIM License
 " Maintainer:	Nirbheek Chauhan <nirbheek.chauhan@gmail.com>
 "		Liam Beguin <liambeguin@gmail.com>
-" Last Change:	2016 Dec 7
+" Last Change:	2021 Aug 16
 " Credits:	Zvezdan Petkovic <zpetkovic@acm.org>
 "		Neil Schemenauer <nas@meson.ca>
 "		Dmitry Vasiliev
@@ -118,6 +118,7 @@ syn keyword mesonBuiltin
   \ summary
   \ target_machine
   \ test
+  \ unset_variable
   \ vcs_tag
   \ warning
   \ range

docs/markdown/Reference-manual.md

@@ -1852,6 +1852,17 @@ Defined tests can be run in a backend-agnostic way by calling
 `meson test` inside the build dir, or by using backend-specific
 commands, such as `ninja test` or `msbuild RUN_TESTS.vcxproj`.
 
+### unset_variable()
+
+*(since 0.60.0)*
+
+```meson
+    void unset_variable(varname)
+```
+
+Unsets a variable. Referencing a variable which has been unset is an error until
+it has been set again.
+
 ### vcs_tag()
 
 ``` meson

docs/markdown/snippets/unset_variable.md

@@ -0,0 +1,16 @@
+## `unset_variable()`
+
+`unset_variable()` can be used to unset a variable. Reading a variable after
+calling `unset_variable()` will raise an exception unless the variable is set
+again.
+
+```meson
+# tests/meson.build
+tests = ['test1', 'test2']
+
+# ...
+
+unset_variable('tests')
+
+# tests is no longer usable until it is set again
+```

mesonbuild/ast/interpreter.py

@@ -127,6 +127,7 @@ class AstInterpreter(InterpreterBase):
                            'subdir': self.func_subdir,
                            'set_variable': self.func_do_nothing,
                            'get_variable': self.func_do_nothing,
+                           'unset_variable': self.func_do_nothing,
                            'is_disabler': self.func_do_nothing,
                            'is_variable': self.func_do_nothing,
                            'disabler': self.func_do_nothing,

mesonbuild/interpreter/interpreter.py

@@ -360,6 +360,7 @@ class Interpreter(InterpreterBase, HoldableObject):
                            'static_library': self.func_static_lib,
                            'both_libraries': self.func_both_lib,
                            'test': self.func_test,
+                           'unset_variable': self.func_unset_variable,
                            'vcs_tag': self.func_vcs_tag,
                            'range': self.func_range,
                            })
@@ -2638,6 +2639,16 @@ This will become a hard error in the future.''', location=self.current_node)
     def func_is_variable(self, node: mparser.BaseNode, args: T.Tuple[str], kwargs: 'TYPE_kwargs') -> bool:
         return args[0] in self.variables
 
+    @FeatureNew('unset_variable', '0.60.0')
+    @typed_pos_args('unset_variable', str)
+    @noKwargs
+    def func_unset_variable(self, node: mparser.BaseNode, args: T.Tuple[str], kwargs: 'TYPE_kwargs') -> None:
+        varname = args[0]
+        try:
+            del self.variables[varname]
+        except KeyError:
+            raise InterpreterException(f'Tried to unset unknown variable "{varname}".')
+
     @staticmethod
     def machine_from_native_kwarg(kwargs: T.Dict[str, T.Any]) -> MachineChoice:
         native = kwargs.get('native', False)

mesonbuild/interpreterbase/interpreterbase.py

@@ -542,7 +542,7 @@ The result of this is undefined and will become a hard error in a future Meson r
         func_name = node.func_name
         (h_posargs, h_kwargs) = self.reduce_arguments(node.args)
         (posargs, kwargs) = self._unholder_args(h_posargs, h_kwargs)
-        if is_disabled(posargs, kwargs) and func_name not in {'get_variable', 'set_variable', 'is_disabler'}:
+        if is_disabled(posargs, kwargs) and func_name not in {'get_variable', 'set_variable', 'unset_variable', 'is_disabler'}:
             return Disabler()
         if func_name in self.funcs:
             func = self.funcs[func_name]

test cases/common/244 variable scope/meson.build

@@ -0,0 +1,15 @@
+project('variable scope')
+
+x = 1
+
+assert(is_variable('x'))
+
+assert(get_variable('x') == 1)
+
+set_variable('x', 10)
+
+assert(get_variable('x') == 10)
+
+unset_variable('x')
+
+assert(not is_variable('x'))